Normal and abnormal extraocular muscle will be examined in serial sections of whole epon-embedded muscles. In normal muscle we plan to (a) discriminate morphologically distinct fiber populations and their innervation, (b) clarify distinctions among neuromuscular junctions of multiply-innervated fibers, (c) define the topographical organization of respective extraocular muscles in terms of component fiber populations. In abnormal muscle we plan to study (a) animal models inbred for disorders resembling Duchenne and myotonic dystrophy, with emphasis on the time-course analysis of structural changes, (b) rats and mice exhibiting drug induced myotonia, with emphasis on relating reversibility of the syndrome with structural correlates. The procedure provides for light microscopic examination of thick epon sections for the detection of areas of interest, including the location of neuromuscular junctions. Selected thick sections are reembedded for further electronmicroscopic examination, in either the transverse or longitudinal planes of section.